The cellular and molecular effects of the combination of psoralen and long wavelength ultraviolet light (UV-A) are being studied. This combination, which induces DNA-psoralen binding is being used experimentally for treatment of psoriasis and the cutaneous lymphoma mycosis fungoides. Psoralen plus UV-A has been found to be mutagenic in bacteria and carcinogenic in mice. There is a significant decrease in DNA synthesis in circulating leucocytes from patients following this treatment. The present research involved development of an in vitro assay to measure the effects of psoralen plus UV-A on cell proliferation and DNA synthesis using rapidly dividing human lymphoblastoid cells. These studies may be useful in governing the use of psoralen plus UV-A by indicating conditions of toxicity and possible mutagenicity to human cells and in clarifying the mechanism of cell damage by psoralen.